As is known in the art, it is sometimes desirable to provide a switch between an input section and an output section of a microwave transmission line. One device for performing this function is a radio frequency (RF) micro-electro-mechanical system (MEMS) switch. This existing type of switch typically has a substrate with two conductive posts spaced apart on the substrate. The switch includes a switchable capacitor having a conductive part (e.g., a bottom electrode of the switchable capacitor) on the substrate disposed between, but electrically separated from, the bottom portions of the posts. The bottom electrode is covered by a layer of a solid dielectric material. A flexible, electrically conductive membrane (e.g., the upper electrode of the switchable capacitor) extends between, and has ends thereof electrically connected to, the tops of the posts, so that a central portion of the flexible, electrically conductive membrane is suspended above the bottom electrode. An input section of a microwave transmission line is coupled to one of the first and second electrodes and an output second on the transmission line is coupled to the other one of the transmission line.
An RF signal is typically applied to the input section and is capacitively coupled through the switch to the output section. More particularly, the switch includes a switchable voltage source coupled between the top and bottom electrode for producing a switchable electrostatic force between the first electrode and to second electrodes (i.e., between the bottom and top electrodes). The switchable electrostatic force changes the spacing between the first and second electrodes, and hence switches the capacitance of the switch, selectively in accordance with a voltage produced by the switchable bias voltage source.
More particularly, in the absence of a voltage produced by the switchable voltage source, (i.e., in a de-actuated or non-actuated state), the second, flexible electrode is spaced above both the first electrode and the solid dielectric layer in the low capacitance state. In order to actuate the switch, a switchable voltage source is connected between the first and second electrodes. This bias voltage produces electrostatic charges on the first and second electrode and the charges cause the first and second electrodes be electrostatically attracted to each other. This attraction causes the central portion of the second, flexible electrode to move downwards towards the first electrode and touches the top of the solid dielectric layer; this is the actuated position or high capacitive state of the switch.
In this actuated state of the switch, the spacing between the first and second electrodes is less than in the de-actuated state. Therefore, in the actuated state, the capacitive coupling between the first and second electrodes is significantly larger than in the de-actuated state. Consequently, in the actuated state, the RF signal traveling through one of the first and second electrodes is capacitively coupled substantially in its entirety to signals traveling along the other one of the first and second electrodes.
In order to de-actuate the switch, the DC bias voltage is turned off. The inherent resilience of the second, flexible electrode then returns to its original position, which represents the de-actuated state of the switch. Because the capacitive coupling between the first and second electrodes is much lower in the de-actuated state, the RF signal traveling through one of the first and second electrodes experiences little or no capacitive coupling to signals traveling along the other one of the first and second electrodes.
The inventors have recognized that in certain applications, it is desirable to have a high pull down voltage on the second, flexible electrode for high power applications since if the pull down voltage is too low; the RF, microwave signal can itself activate the switch; an undesirable effect. If the pull down voltage on conventional RF MEMS switch is increased, this will increase the electric field strength in the solid dielectric when the MEMS switch is closed (i.e., in the activated state). This can result in solid dielectric breakdown or excessive charging of the solid dielectric which can lead to “stiction” of the second, flexible electrode which can cause the flexible membrane to remain in the down or closed position even after the DC bias voltage is turned off and the switch is to return to the deactivated state.
In accordance with the present disclosure, a switchable capacitor is provided having: a solid dielectric; a pair of electrodes, a first one of the electrodes having the solid dielectric thereon and a second, flexible one of the electrodes suspended over the solid dielectric when the switchable capacitor is in an de-activated state; and a top plate disposed between the solid dielectric and the second, flexible electrode and connected to a reference potential. The top plate is coupled to a reference potential. When the switchable capacitor is electrostatically driven to an activated state, the second, flexible one of the electrodes contacts the top plate, and when the switchable capacitor is returned to the de-activated state, charge on the top plate is discharged to the reference potential.
In one embodiment, a switch is provided having: a solid dielectric; a pair of electrodes, a first one of the electrodes having the solid dielectric thereon and a second, flexible one of the electrodes being suspended over the solid dielectric when the switchable capacitor is in a de-activate state; and an aperture top metal or resistive plate disposed between the solid dielectric and the second, flexible electrode and connected to a reference potential. The top metal or resistive plate is fabricated with multiple holes through which the solid dielectric is exposed. The structure includes a switchable voltage source coupled between the first electrode and the second electrode for producing a switchable electrostatic force between the first electrode and the second electrode to electrostatically drive the capacitor between an activated state and the de-activated state. When the switchable capacitor is electrostatically driven to the activated state, the second one of the electrodes contacts the top plate. When this occurs, the top plate and second electrode are equipotential and thus no electrostatic force exists between them. Electrostatic force remains between the top plate and the bottom electrode through the apertures in the top plate. The force applied to the top electrode is a function of the applied voltage and the combined area of the apertures. The advantage here is charges trapped below the top plate are shielded by the top plate and will have no impact on the activation voltage of the flexible second electrode. If while activated, the solid dielectric traps a charge where the top plate is aperture this charge will be minimal and not sufficient to hold down the second electrode. Upon release of the second electrode any charge remaining in the solid dielectric under the top plate will terminate at the top plate and thus the second electrode will only see the potential of the top plate and not the charge within the solid dielectric. A discharge path is provided between non-apertured portion of the top plate and a reference potential to remove charge on the top plate when the switchable capacitor is returned to the de-activated state.
In one embodiment, a switching system is provided having: a substrate; a microwave transmission line having an input section and an output section; and a switchable capacitor disposed on the substrate. The capacitor includes: a solid dielectric; a pair of electrodes, a first one of the electrodes having the solid dielectric thereon and a second, flexible one of the electrodes being suspended over the solid dielectric when the switchable capacitor is in an de-activated state and wherein the second, flexible one of the electrodes is electrostatically driven toward the first electrode when the capacitor is switched to an activated state; and a top plate disposed between the solid dielectric and the second, flexible electrode and connected to a reference potential. A switchable voltage source is coupled between the first electrode and the second electrode for producing a switchable electrostatic force between the first electrode and the second electrode to electrostatically drive the capacitor between the activated state and the de-activated state. The voltage source switches between an activation voltage and a lower de-activation voltage. When the switchable capacitor is electrostatically driven to the activated state: microwave energy on the input section is coupled to the output section through the switchable capacitor; and the second one of the electrodes contacts the top plate and charge on the second one of the electrodes is discharged to a voltage less than the activation voltage of the switchable voltage source, thereby limiting the voltage across the solid dielectric layer.
In one embodiment, the top plate is resistive and a circuit is provided to heat the resistive top plate.
In one embodiment the circuit includes the resistive top plate.
The details of one or more embodiments of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the disclosure will be apparent from the description and drawings, and from the claims.
Like reference symbols in the various drawings indicate like elements.